Energy costs and end-use consumption in the United States has risen dramatically over the past few years. Electricity, which accounts for almost half of the end-use energy consumption in the US, is highly dependent on the price of fossil fuels. With the rising demand and cost comes a requirement for more efficient and cost effective power and, in order to reduce dependence on fossil fuels, improvements in energy efficiency are needed.
Superconductor materials have been known and understood by the technical community for a long time. It was not until 1986, however, that the first oxide-based high-temperature (high-Tc) conductor with superconductive properties at a temperature above that of liquid nitrogen (77 K) was discovered, namely YBa2Cu3O7 (YBCO). Since then, additional high-Tc superconductor (HTS) materials, such as Bi2Sr2Ca2Cu3O10+y (BSCCO) and others, have also been developed.
The more recent development of HTS materials provides potential for the economically feasible manufacture of superconductor components. Perhaps most significantly is the development of such materials for use in the power industry, including applications for power generation, transmission, distribution, and storage.
In particular, more efficient electric power systems depend on more efficient wire technology. Thus, the development and use of HTS materials has led to the development of new wire technology. HTS wire can carry significantly more current than conventional copper and aluminum conductors of the same physical dimension, and it offers major size, weight, and efficiency benefits. HTS superconductors in the power industry include an increase in one to two orders of magnitude of power-handling capacity, significant reduction in the size (i.e., footprint) of electric power equipment, reduced environmental impact, greater safety, and increased capacity over conventional technology.
Accordingly, HTS technologies can drive down costs and increase the capacity and reliability of electric power systems. Developments in HTS wire will offer a powerful tool to improve the performance of power grids while reducing their environmental footprint.
However, many challenges exist with the commercialization of HTS superconductors. In particular, one obstacle has been the fabrication of a commercially feasible HTS tape that can be used to form the various power components. The “first generation” of HTS tapes included use of the above-mentioned BSCCO high-Tc superconductor, typically embedded in a matrix of noble metal (e.g. Ag). Due to materials and manufacturing costs, however, such tapes do not represent a commercially feasible product for the extended lengths need.
More recently, “second generation” HTS tapes have been shown to have superior commercial viability. These tapes generally are comprised of a layered structure, typically including a flexible substrate that provides mechanical support; at least one buffer layer overlying the substrate; an HTS layer overlying the buffer film; and at least one stabilizer layer, typically formed of a nonnoble metal, overlying the HTS layer. Generally, more than one stabilizer layer is utilized and a capping layer, typically a noble metal layer, is deposited along the entirety of the HTS tape, including its side surfaces, in order to isolate the sensitive HTS layer from reacting with the non-noble stabilizer layer.
One such second-generation HTS tape utilizes YBCO coated conductor technology. The architecture of the coated conductor has not been optimized for AC applications, however, such as motors, generators, and transformers. In particular, the width and thickness of HTS tapes have a high dimension ratio, which in turn results in HTS coated conductors exhibiting very high hysteretic losses. Prior art discloses that magnetization losses can be reduced if the superconducting layer is divided into many parallel superconducting stripes segregated by nonsuperconducting resistive barriers. Therefore, in order to minimize AC hysteretic losses, it is desirable to subdivide the HTS layer of a tape into long thin linear filamentary stripes, or striations.
Although these multifilament conductors have been shown to greatly reduce hysteretic losses, numerous engineering and manufacturing challenges remain prior to full commercialization of striated second generation-tapes. A significant reduction of magnetization losses in coated conductors is a prerequisite for their use in AC power applications, such as transformers, generators, and motors. It is also important for any modifications to be compatible with current techniques of manufacturing the coated conductors, and it is essential to control the flux and current distributions in the HTS tape.
To date, a method does not exist for fabricating an HTS wire encapsulated with a thick stabilizer layer that also incorporates a striated HTS layer to reduce hysteric losses. As shown in FIG. 1A, conventionally, HTS tapes are fabricated by first depositing a flexible substrate (12). Next, at least one buffer layer (14) is deposited over the substrate, followed by an HTS layer (18), which overlies the buffer layer. A thin protective capping layer (13), typically formed of a noble metal such as Ag, is then deposited over the HTS layer. Finally, a stabilizer layer (19), typically formed of Cu or other non-noble metal, is deposited on the noble metal capping layer. The presence of a thicker, more robust stabilizer layer is beneficial because it provides overcurrent protection and quench stability, and it gives better protection to the HTS tape (rather than just a thin layer of Ag) in subsequent steps when the final non-conductive layer is applied.
At this point, conventional methods would call for photolithographic or other pattern transfer techniques to be used to transfer the desired filament pattern onto the stabilizer layer, followed by the use of wet etching or laser ablation techniques to form the desired striations in the stabilizer and HTS layer. As shown in FIGS. 1B-1C, this striation technique has been used to create multifilament HTS wire when only a thin capping layer is present. However, in order to provide adequate overcurrent current-handling capacity and quench stability, it is desirable to use both a thin, noble metal capping layer and thick, non-noble metal stabilizer layer, both of which completely encapsulate the HTS wire. In this way, the architecture of the HTS wire is better optimized for practical AC applications.
The thickness of the stabilizer layer, however, is generally within a range of about 1-1000 microns, and most typically within a range of about 10 to about 400 microns. Forming striations through the thick stabilizer layer using known chemical-etching techniques is prohibitively laborious, such that the fabrication of a multifilament HTS tape with a thick stabilizer layer has not yet been possible. In particular, etching through a thick stabilizer layer in addition to the thin capping layer, before reaching the HTS layer, results in much longer period of contact with a particular etching agent. A longer etching contact time causes an increase in the degradation of the HTS materials and a loss of superconducting properties due to an increased chemical reaction. Accordingly, it has not been feasible to use state of the art etching techniques in order to form multifilament HTS tapes with thick stabilizers.